I am not sure there is an exersize that will work. The double chin is composed of fat, and would be one of the last places to disappear even with stringent dieting. Unfortunately I think liposuction is the only answer.

I remember reading about a double chin exercise. Sit up straight in an armless chair, holding the bottom of the chair with your hands at your sides, tilt your head back and alternately close your mouth pushing your chin forward and them open your mouth as wide as you can. How many reps? I don't recall.

Here is another:

Correcting Flabbiness Under Chin

Step 1 Lift chin slightly upward.

Step 2 With flat point of index and middle fingers press against indentation (hollow) formed by the two upper nodules of the larynx located at the curve from chin to neck. Do not be concerned in the beginning as to the precise location of pressure which you will discover through practice. Regard finger pressure as the resistance against the working muscles.

Step 3 Now press your back teeth firmly together * and while maintaining this position,

Step 4 Press the tip of your tongue against the inside of lower front teeth (gum line) increasing pressure with the tip of tongue ** in 10 or more gradual and definite steps (Count to 10).

Step 5 Hold end position of pressure for 6 seconds (count slowly to six).

Although as we get older our skin and muscles tend to become more lax, this doesn't mean that a double, sometimes triple, chin is inevitable.

Just like any other part of the body, the tissue under the chin collects fat if it's given the opportunity. This is the major cause for double chins; simple fat deposition.

The first step is to ensure your diet is high in fibre and low in fat and calories, containing plenty of fresh fruit and vegetables. This will help prevent you laying down vast amounts of fat.

Using a skin toner everyday helps the skin remain firm, as does slapping under the chin with the back of your hand. When slapping, start gently and build up speed gradually. Do this for a couple of minutes each time, two or three times a day.

The next step is to exercise the muscle called the platysma whose function is to pull down the corners of the mouth and pull down the jawbone. Do the following exercise within the privacy of your own home if you're not comfortable with people looking at you oddly.

Open your mouth wide, and then pull up your bottom lip over your bottom teeth. Move your jaw up and down as if you're trying to scoop something up with it - imagine the movement of the shovel on a digger scooping up earth or debris on a building site. Perform ten repetitions of this exercise each time, and again do this two or three times a day.